1. Technical Field
This invention relates to an electromagnetic balance-type weighing apparatus for measuring the weight of an object of weighing by detecting a signal designed to automatically generate an electromagnetic force by means of an electromagnetic coil to offset the displacement of the weighing pan (or balance beam) of the apparatus caused by the weight of the object of weighing and, more particularly, it relates to an improvement realized on such a weighing apparatus for fast-responsiveness, enhanced accuracy and hence high reliability.
2. Background Art
Weighing apparatus of a conventional differential transformer type and load cell type used in various manufacture and process lines for weighing articles (works) have gradually been replaced by those of electromagnetic balance-type as disclosed in U.S. Pat. No. 4,300,647.
The configuration of a known typical electromagnetic balance-type weighing apparatus is shown in FIG. 4 of the accompanying drawings.
In FIG. 4, reference numeral 1 generally denotes a weighing pan/balance beam assembly supported at fulcrum S selected to show a given leverage, the weighing pan being positioned at one end portion 1a of the balance beam for determining the weight of object w placed on it, reference numeral 2 denotes a circular hole provided at a bent-down section of the other end portion 1b of the balance beam of the assembly 1 and reference numeral 3 denotes a light emitter that emits beams of light toward the hole 2, while reference numerals 4 and 5 respectively denote upper and lower light sensors arranged to receive the beams of light passing through the hole 2.
Also, in FIG. A reference numeral 6 denotes a differential amplifier for detecting the difference between the beams received by the light sensor 4 and those received by the light sensor 5. Reference numeral 7 denotes a proportional integral derivative (hereinafter referred to as PID) operation unit for performing the operation of scalar multiplications, integrations and differentiations on the differential signal it receives and sending out a control voltage determined on the basis of the received signal and reference numeral 8 denotes a driving amplifier for generating a driving voltage corresponding to the received control voltage in order to produce a large electric current.
Reference numeral 9 denotes an electromagnetic (force) coil designed to pull the other end portion 1b of the balance beam opposite to the weighing pan downward by means of the electromagnetic force generated by the electric current running through it in accordance with the driving voltage, reference numeral 10 denotes a buffer amplifier for transmitting the voltage generated at resistor R by the electric current running through the electromagnetic coil 9 and reference numeral 11 denotes a low-pass filter (hereinafter referred to as LPF) for producing a DC voltage signal corresponding to the weight of the object of weighing obtained by removing those components of the signal transmitted from the buffer amplifier 10 that are responsible for oscillation and noise.
When the weighing pan 1 of an electromagnetic weighing apparatus having a configuration as described above is displaced clockwise by the weight of an object being weighed, a differential signal is generated by the difference of the amount of light received by the light sensor 5 and that received by the sensor 4, the former now being larger than the latter, so that consequently the control voltage transmitted from the PID operation unit 7 is increased to generate a force to move the weighing pan 1 counterclockwise to restore its initial position and hence the balanced condition of the weighing pan 1.
Since the force of pulling down the end of the balance beam opposite to the weighing pan generated by the electromagnetic coil 9 is linearly proportional to the driving current. The voltage present at the opposite end of the resistor R when the weighing pan 1 is balanced is a linear function of the weight of the object being weighed.
Thus, the weight of the object of weighing can be determined by detecting the output signal of the LPF 11, once the voltage output of the opposite end of the resistor is calibrated for the load of the weighing apparatus.
However, if the load applied to a conventional electromagnetic balance-type weighing apparatus as described above by an object of weighing changes stepwise and the electromagnetic coil 9 is driven to generate electromagnetic force by a voltage applied to it, the electric current caused to run through the electromagnetic coil 9 as a transient response to the inductance of the coil returns to its normal level very slowly following the curve of natural logarithm, a phenomenon that requires a long period of time for weighing an object.
This phenomenon can give rise to a very serious problem of extremely low weighing speed particularly when the weighing pan is realized in the form of a conveyor belt that moves objects of weighing on it at a given pitch and a given rate up to the weighing spot in an attempt to maximize the rate or efficiency of the weighing operation. While this problem may be dissolved to some extent by increasing the pitch of differentiation in the PID operation unit 7, such a modification made to the operation unit 7 can prohibit optimization of the control capability of the apparatus which is the principal objective of the differentiation and, at the same time, eventually cause the control system of the apparatus to oscillate and consequently deteriorate its operation.
As seen from FIG. 22 that schematically illustrates the principle of a known electromagnetic balance-type weighing apparatus as described above, the apparatus receives a force F1 at an end of the balance beam L applied by the object of weighing W to the point of weighing of the weighing pan C (realized in the form of a weighing conveyor belt for instance) located at that end of the beam L, which is supported at the fulcrum S, and a sucking force F2 of the electromagnetic coil FC at the other end of the beam L in such a way that the beam L is balanced by the two forces so that the weight of the object of weighing W on the weighing pan C can be determined from the electric current supplied to the electromagnetic coil FC to maintain the balanced condition.
If the weighing pan C (realized in the form of a weighing conveyor belt) is cleared of any object of weighing W, certain electric current should be supplied to the electromagnetic coil FC at a given rate to counterbalance the dead weight of the weighing pan C and keep the balance beam L horizontal by pulling the other end A of the balance beam L downward around the fulcrum S, because otherwise the other end A of the balance beam L will be turned upward by the dead weight of the weighing pan C. When an object of weighing W is placed on the weighing pan C, the electric current supplied to the electromagnetic coil FC should be augmented to increase its sucking force F2 in order to offset the upward movement of the other end A of the balance beam L and keep its balanced condition. The weight of the object of weighing w is determined from the increase in the rate of electric current supplied to the electromagnetic coil FC.
A known electromagnetic balance-type weighing apparatus as described above is, however, accompanied by the problems as described below.
(a) Since the dead weight of the weighing pan C is counterbalanced by the sucking force F2 of the electromagnetic coil FC, the latter should be constantly fed with electricity even when the weighing pan C does not have any object w on it in order to hold the balance beam L horizontal and therefore the level of power consumption of such an apparatus is not negligible.
When the weighing pan C is realized in the form of a conveyor belt that moves objects of weighing W at a given pitch and a given rate, the dead weight to be offset by the sucking force of the electromagnetic coil FC includes the weight of the conveyor belt and that of the electric motor for driving the conveyor belt and therefore the power requirement of such an apparatus can be considerable. This in turn gives rise to a high rate of heat generation by the electromagnetic coil and the high temperature caused by the generated heat can adversely affect the accuracy of operation of the electromagnetic coil and therefore that of the electromagnetic balance-type weighing apparatus.
(b) In order to accommodate any heavy object on the weighing pan C and keep the balanced condition of the beam L by means of the sucking force F2 of the electromagnetic coil FC, the ratio of the distance l1 between the fulcrum S of the balance beam L and the point of weighing of the weighing pan C to the distance l2 between the fulcrum S and the point of receiving the sucking force of the electromagnetic coil FC should be strongly in favor of the latter. Because of the inversely high ratio of l1 to l2 and the remarkable weight imbalance between the weighing pan C side and the other side of the balance beam L as viewed from the fulcrum S, the balance beam L is highly sensitive to and can resonate with external vibrations (such as the vibration of the floor where the weighing apparatus is installed), that can also adversely affect the accuracy of operation of the apparatus.
Besides, the balance beam L can significantly vibrate when an object of weighing W is placed on or removed from the weighing pan C because of an abrupt weight imbalance there, a phenomenon that also adversely affects the accuracy of the weighing operation of the apparatus.
FIG. 23 of the accompanying drawings schematically illustrates the configuration of a known water-proof weighing apparatus using a differential transformer.
A weighing pan C (in the form of a conveyor belt for instance) is arranged external to a housing B of the apparatus main body and at one end portion of the balance beam L which is separated from the fulcrum S by a short distance for receiving an object of weighing on a part of it.
Supports Ca, Ca for supporting the weighing pan C stand upright through respective holes Ba, Ba of the housing B in such a manner that the weighing pan C can freely move upward and downward. A spring SP is rigidly fitted to the other end portion of the balance beam L as viewed from the fulcrum S and the balance of the beam L is secured by the resilient deformation of the spring SP. The spring SP acts also as a sensor spring that provides (angular) displacement of the balance beam L with a magnitude which is proportional to the weight of the object of weighing W on the weighing pan C. With such an arrangement, the weight of the object of weighing W can be determined by detecting the displacement of the balance beam L by means of a differential transformer T whose core TC is arranged at the one end portion of the balance beam L opposite to the weighing pan C.
If the object of weighing W being weighed on a weighing apparatus having a configuration as described above contains liquid in it, some of the liquid content can flow into the main body of the weighing apparatus through the whatever small space between the holes Ba, Ba of the housing B and the respective supports Ca, Ca. A similar problem may arise when the weighing pan C is washed with water.
In order to eliminate such a problem, conventionally a cylindrical protector hood Pa is fitted around each of the supports Ca, Ca and a water-proof cylinder Pb is arranged along the edge of each of the holes Ba, Ba of the housing B, as shown in FIG. 23. A drain ridge Pc is also arranged around the outer periphery of the top of each of the water-proof cylinders Pb, Pb whenever necessary.
However, a water-proof weighing apparatus of the type as described above is accompanied by the following problems.
(a) while the clearance between each of the drain ridges Pc, Pc of the water-proof cylinders Pb, Pb and the corresponding water-proof hood Pa should be minimized in order to effectively prevent liquid from flowing into the housing B, it should be large enough to avoid any possible contact between the supports Ca, Ca and the respective water-proof cylinders Pb, Pb, hoods Pa, Pa and the drain ridges Pc, Pc that can be brought about by dust and/or misalignment of the supports Ca, Ca and the respective holes and secure free movement of the supports Ca, Ca. A large clearance by turn hinders complete protection of the weighing apparatus against liquid.
(b) Since the inside of the casing is exposed to atmosphere, dew can be formed within the weighing apparatus.
While the space between the housing B and the supports Ca, Ca may be airtightly covered by elastic members D, D as illustrated in FIG. 24, such an arrangement is also accompanied by the following drawbacks.
(c) The resilient force of the elastic members D, D applied to supports Ca, Ca adversely affects the operation of the weighing apparatus so that the displacement of the balance beam L does not correctly represent the weight of the object W being weighed.
(d) Since the housing B is airtightly sealed, the internal air pressure of the housing B is changed by the temperature of the inside of the housing B to consequently give rise to errors in measurement.
(e) Since the casing B is airtightly sealed, the air contained in the housing B acts as a cushion that can unintentionally vibrate the balance beam L, requiring some time before the beam returns to a stationary condition.
It should be noted that similar problems are observed with any conventional electromagnetic balance-type weighing apparatus, where the differential transformer T is replaced by an electromagnetic coil FC fitted to one end portion of the balance beam L as illustrated in FIG. 22.
In short, any existing electromagnetic balance-type weighing apparatus is devoid of fast-responsiveness and does not meet the requirement of high precision and high reliability.